From the Authors:
Dr. Kawada has raised an important question about the sensitivity setting of the actigraphy used in our study (1). For this study, we chose to use the medium sensitivity threshold. This decision was based on the findings of Meltzer and colleagues, the only published study to directly compare the Actiwatch 2 (Philips Respironics, Bend, OR) to overnight polysomnography in school-aged children, including those with and without sleep apnea (2). This allowed for the greatest estimate of sensitivity and accuracy. Although the “low” threshold setting would have allowed for greater specificity, this comes at a cost to both sensitivity and accuracy for school-aged children and across levels of sleep-disordered breathing (2).
In general, actigraphy is well known to be highly sensitive to detecting sleep, but rather poor in its specificity or ability to detect wake after sleep onset. In fact, in a review of 228 studies that used actigraphy in pediatric populations, more than half of the validation studies demonstrated a specificity of less than 0.60 (3). This was also true in our study (1).
Dr. Kawada cited several studies that have examined the validity of actigraphy (4–6). However, it should be noted that each of these studies included only adult patients with sleep disorders, including insomnia. Because adults can often lie still for extended periods of time without moving (i.e., motionless wakefulness), actigraphy provides a poor estimate of sleep time in adults. In particular for patients with insomnia, actigraphy will overestimate total sleep time and underestimate wake after sleep onset. Furthermore, the one pediatric study cited by Dr. Kawada included preschoolers (younger than the population we studied) and compared actigraphy to videosomnography rather than overnight polysomnography (7). Not only are additional validation studies needed that compare actigraphy to polysomnography, but it is important for researchers to report all of the relevant settings when using actigraphy (3).
Footnotes
Author disclosures are available with the text of this letter at www.atsjournals.org.
References
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